The present invention relates generally to satellite momentum storage devices, and more particularly, to a safing mode for high momentum states in body stabilized spacecraft.
Throughout the life of a spacecraft, attitude modifications are made to carry out mission objectives, to determine orientation, and to correct for undesired torque. In order to minimize expendable fuel in slewing maneuvers, reaction wheel systems are used to transfer rotational momentum to and from the satellite body.
When a fully deployed 3 axis controlled spacecraft accumulates angular momentum beyond the control system""s ability to maintain pointing, prompt action is necessary to ensure that the spacecraft will survive. The ability of satellites with ion propulsion to quickly accumulate angular momentum is limited. However, the ability of satellites with chemical propulsion systems to quickly accumulate angular momentum is a real danger. In the presence of this danger it is critical to devise a method of autonomous spacecraft safing that can control a spacecraft while maintaining power and thermal survivability.
Prior art methods do not have the ability to autonomously safe a spacecraft in the presence of large angular momentum. Some prior art methods must store large amounts of angular momentum diurnally and also use a chemical propulsion system. In many prior art Fault Hold modes, the spacecraft body was commanded to rotate continuously about an axis transverse to the solar wing axis. This rotation is termed xe2x80x9crotisseriexe2x80x9d. This rotation is done using a reaction wheel array for the actuators, and gyros for the sensors. The solar wings then rotate about the solar wing drive axis to point at the sun. Solar wing drive motors are used as actuators, and wing current sensors as the sensors. The body slew direction and sign are chosen to take the best advantage of the wheel array capability, and to use the system momentum to advantage. However, for certain system momentum directions and magnitudes, the desired slew could not be maintained, due to reaction wheel saturation.
Additionally, in many cases, the prior art methods are incapable of producing stable rotation about a desired axis when the total system momentum is greater than or equal to the storage capability of the momentum storage/exchange devices. In some cases, the prior art is capable of spinning a body about a desired axis, but does not account for power survival, low momentum survival and minimum spin speed requirements.
The disadvantages associated with these conventional satellite safing techniques have made it apparent that a new technique for satellite safing is needed. The new technique should allow autonomous spacecraft safing that can control a spacecraft while maintaining power and thermal survivability. Additionally, the new technology should allow autonomous spacecraft safing in the presence of large system angular momentum that has a much larger momentum envelope than previous methods. The present invention is directed to these ends.
It is, therefore, an object of this invention to provide an improved and reliable safing mode for high momentum states in body stabilized spacecraft. Another object of the invention is to allow autonomous spacecraft safing that can control a spacecraft while maintaining power and thermal survivability. An additional object of the invention is to allow autonomous spacecraft safing in the presence of large system angular momentum that has a much larger momentum envelope than previous methods.
In accordance with the objects of this invention, a safing mode for high momentum states in body stabilized spacecraft system is provided. In one embodiment of the invention, a spacecraft with a reaction wheel system can be autonomously safed by setting the solar wings to continuous tracking, determining a slew rate vector based on the total angular momentum, and slewing the spacecraft using the slew rate vector until commanded to stop autonomous safing. When the total angular momentum of the spacecraft is to large to be handled by rotisserie, then the spacecraft is reoriented to align a suitable rotation vector with the system momentum. In a typical application, the spacecraft has a reaction wheel assembly with four wheels arranged to form a right regular pyramid. Two reaction wheels on opposite edges of the pyramid form a first pair and the two remaining reaction wheels forming a second pair. The slew axis of rotation is found by determining as a selected pair the first pair if either reaction wheel in the second pair is inoperative, otherwise determining as the selected pair the second pair and determining as the slew axis of rotation the normalized projection of the axes of rotation of the selected pair onto the base. The slew direction is determined by the sign of the total angular momentum component along the slew axis of rotation.
The present invention thus achieves an improved safing mode for high momentum states in body stabilized spacecraft. The present invention is advantageous in that it is capable of producing stable rotation about a desired axis when the total system momentum is greater than or equal to the storage capability of the momentum storage devices while accounting for power and low momentum survival and minimum spin requirements.
Additional advantages and features of the present invention will become apparent from the description that follows and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims taken in conjunction with the accompanying drawings.